Get ready for some serious elbow grease!!
If your car still has the original paint (IE, no subsequent coats), and the body is straight, then do not take it down to bare metal. If the filler was done properly, then don't worry about it.
Get a DA sander (air driven), and sone sanding blocks and a flexible rubber sanding block and a bunch of 320 grit paper. (you will need a good size compressor for the DA if you don't want to stop every 10 minutes for the compressor to catch up. My 5hp (peak) 20 gal really had a tough time. I now have a 60 gal, 7hp (peak, 5hp run) which has no problem.
Clean the car well with degreaser (from the paint store). Then get sanding. When it is all sanded, shoot it with a catalyzed primer surfacer. Sand with 400 grit prior to shooting color.
(BTW, I am right in the middle of repainting the 71 Cheyenne 20 in the driveway, so this is fresh in my mind).
Tim